In this liver function video we’re going to look at the functions of the liver. The liver physiology. The liver has many, many functions. Let’s look at them one by one.

First of all, it can synthesize amino acids. The process, one of the processes is called transamination, where we have an alpha-ketoacid, which is an alpha carbon, with a keto group and a carboxyl group, which can get converted into an alpha amino acid, where the alpha carbon has an amino group and a carboxyl group.

Now the process, as I mentioned, is known as transamination. And the enzyme used is usually known as transaminase or aminotransferase. So let’s look at one example.

One example is if we have glutamic acid here, plus pyruvic acid, with what enzyme will yield alpha ketoglutamic acid, and also alanine? Alanine is the amino acid, where the amino group is transferred from the glutamic acid into the pyruvic acid, making alanine. The enzyme used is known as alanine transaminase, or ALT. Note the transaminase.

The liver can also metabolize amino acids. A process, known as oxidative deamination, essentially deaminates a molecule: removes an amine group, which is NH3 or 2. Because it is an oxidation process, the molecule itself is oxidized, and so hydrogen is removed.

Let’s look at an example. Let’s say we have L-Glutamic acid here with the amino group. And this will be oxidized to a molecule, and then be hydrated again. When it gets hydrated, the previously L-Glutamic acid will liberate an ammonia group. This ammonia group left the L-glutamic acid, and so this process is known as deamination, because the amine group left. Ammonia is important later on because it can enter the urea cycle. We will look at it later on though.

But first, another function the liver possesses is that it can synthesize proteins. It can synthesize many, many proteins. One of the most important proteins is albumin. Actually fifty percent of the total protein the liver synthesizes is albumin. Albumin helps in maintaining the osmolarity and also transportation of various hormones.

The liver also produces, synthesizes, proteins for the immune system, such as the C – reactive protein, opsonin, which helps for opsonization—opsonization of a pathogen—and also it synthesizes plasma complement proteins c1-c9. Complement proteins are important for destroying a invading pathogen.

The liver also synthesizes various hormones and prohormones, such as insulin like growth factor: IGF, not ILF—not ILGF, sorry—it’s IGF. IGF is important for growth and also has anabolic effects. The liver also produces thrombopoietin, which is important in platelet production. Platelets is important for hemostasis, for fixing up blood vessels. The liver also produces another hormone known as angiotensinogen. Angiotensinogen is part of a big system known as renin-angiotensin-aldosterone system. This system helps regulate blood pressure.

The liver also synthesizes proteins that are necessary—that are clotting factors and inhibitors of coagulation, so anticoagulants. These are prothrombin, fibrinogen, and also antithromben, and alpha 2-macroglobulin. Prothrombin and fibrinogen you may have heard from hemostasis, in the repair of a damage blood vessel, for example.

Now, the liver also synthesizes plasma proteins that are—or carrier proteins—that help carry substances, or molecules, from A to B, such as from the liver to the kidney, or from the, or from the liver to the muscle, for example. So these are carrier proteins. An example of carrier proteins is transferrin. Transferrin carries iron ions in the ferric acid forms: Fe3+. So transferrin carries iron ions around the body. Another one, carrier protein, is IGF: insulin like growth factor binding protein, and this carries insulin like growth factor one, which if you remember is also produced by the liver. And, if you remember insulin like growth factor one is important for growth and development.

Now finally we can go back to another function the liver does is that, is, urea, the production of urea. So the liver produces urea. What is urea? Well, let’s just draw the liver here. So, just recapping, we know that amino acids can be made into proteins, can synthesize proteins. It can synthesize plasma proteins. Plasma proteins such as transferrin and insulin like growth factor carrier protein, which are both carrier proteins: they carry proteins from A to B around the body. Amino acids can also be enter conversion, can enter into conversion, so it can become a pyruvate or it can become another sort of molecule, but not amino acids.

Alternatively, amino acids can follow deamination—removement of the ammonia group, and so we have an accumulation of ammonia. Nucleic acids can also be metabolized to give off ammonia. So what happens with this ammonia? Well ammonia, NH3, can enter the urea cycle, what’s called the urea cycle, to produce urea. Urea will then get expelled by the body. And this is how amine groups get expelled, NH groups get expelled from the body, through urea, and they’re usually excreted in urine, so they go to the kidneys.

Bile production

Now possibly, well one of the most important functions the liver has is the production of bile, so bile production. So the liver actually produces bile, and secretes the bile, where it gets stored in the gall bladder. The gall bladder is underneath, right underneath the liver, and looks something like this. And then we have a common bile duct where the gall bladder can essentially excrete, or secrete, out the bile into the intestines to help digest fats.

So let’s look at this in a different diagram. Here we have the intestines, and we have the pancreas, right next to the intestines. The small intestines this is. The gall bladder is situated here and it essentially secretes its substances through the common bile duct, which enters the intestines over here. The gall bladder stores bile. So what happens is, when we eat fatty foods, fatty foods will get digested by the stomach, will enter the small intestines, and will stimulate the small intestines to secrete a hormone known as CCK because the intestines cannot absorb these fatty substances just like that. It needs the help of bile. So it secretes the hormones CCK, which will then stimulate the gall bladder to contract to then release the bile into the small intestines and to help emulsify the fats, cover the fats, so that the fats can be easily digested and absorbed by the small intestines.

Now bile is made up of two main things. It’s made up of bile acids, or salts, and also bile pigments. So bile acids and bile pigments. Let’s firstly look at bile acids.

Bile acids are amphiphilic steroids, and they emulsify ingested fats so that the fats can be easily digested and absorbed by the small intestine. Bile acids are amphiphilic steroids. What does this mean? Well it means that, if this was a bile acid, it would have one side which is hydrophilic, which means it loves water, and the other side which is hydrophobic, which means that it hates water. So what does this mean? Well, when we consume the lipid, and when it’s in the small intestines, the bile acids will begin surrounding it, where the hydrophobic regions will essentially cover the lipid, and the hydrophilic regions will face the outside because the hydrophobic regions are scared of water, and so they face inwards. Whereas the hydrophilic region will face outwards. And they will cover this lipid, essentially emulsify it, and create what’s called a micelles, micelles?

I don’t really know how to pronounce it, but essentially, once they coat this lipid this can get easily digested and absorbed by the small intestines. So some types of bile acids include cholic acid, taurocholic acid, and deoxycholic acid. Those are some type of bile acids and bile salts.

So that was bile acids. Bile acids help emulsify fat.

Bile pigments are different to bile acids, but they are also bile, all together. Bile pigments are actually the breakdown products of hemoglobin. Hemoglobin are the centers of the red blood cell. So they’re the break, they’re the breakdown product of hemoglobin, and these breakdown products, these bile pigments will then be secrete–excreted out in feces, and that is why we have the brown color in our feces.

So, what do I mean by this? So in the blood, if there is extravascular, or intravascular hemolysis, meaning destruction of red blood cells, the outcome of this would be unconjugated bilirubin, which is the product of destruction of red blood cells. These unconjugated bilirubin will travel through the bloodstream bound to albumin, which will then take it to the liver: the hematocytes of the liver.

So here we have now unconjugated bilirubin without the albumin. Unconjugated bilirubin will then be changed to conjugated bilirubin in the liver. Conjugated bilirubin is the bile pigment because it is the product of breakdown of hemoglobin. So this bile pigment, this conjugated bilirubin, will then travel through the biliary system in through the bile duct into the intestines. It’ll get secreted into the intestines. The conjugated bilirubin will then convert to urobilinogen, through bacterial proteolysis. And about ninety percent of the urobilinogen will actually be excreted as feces. So ninety percent of the bile pigments will be excreted as feces, this urobilinogen.

So what happened to the other ten percent? Well the other ten percent will be reabsorbed through the portal vein, where it will then enter back into the liver if you know what the portal vein is. So this urobilinogen will then be in the liver, and will then travel back into the bloodstream towards the kidneys where this ten percent of urobilinogen will be excreted as urine.

So essentially bile has two things. It’s made up of two things: bile acids, or bile salts, and also bile pigments, so bile acids and bile pigments. Bile acids are to help in digestion of fat. Bile pigments are the breakdown products of red blood cells, like urobilinogen or conjugated bilirubin, which then gets excreted by the body as feces mainly. Hope that all makes sense.

Carbohydrate metabolism

Now the other function that a liver does is that it also performs carbohydrate metabolism. Very important. You can also watch this on my biochemistry videos, but we’ll just look at it as an overall picture here. So the liver has a major role in controlling carbohydrate metabolism and so blood glucose levels as well. So for example, after we eat, glucose will be get absorbed by the liver, will get converted to glucose-6-phosphate, then glucose-1-phosphate, and then UDP glucose, and then it will get converted to glycogen. This whole process is known as glycogenesis: the synthesis of glycogen. So glycogenesis would happen if blood glucose levels are high. If blood glucose levels are low glycogen will be broken down into glucose-1-phosphate which will then convert to glucose-6-phosphate and then to glucose, so that glucose can be released into the blood, to increase blood glucose levels. And this process is called glycogenolysis.

Also glucose can convert through a series of reactions to pyruvate and produce ATP in the reaction. If blood glucose levels are low, and glycogen levels are low as well, amino acids and also fats, for example, can be converted to pyruvate, which then will go through a process known as gluconeogenesis to produce more glucose. The opposite of gluconeogenesis is glycolysis. So I hope you can see from this diagram that the liver not only has many functions, such as producing bile, but also in carbohydrate metabolism as well as other macromolecules, such as lipid metabolism. The liver actually produces a lot of lipoproteins. Lipoproteins, as the name suggests, is lipids and proteins, but mainly lipids, which travel around the body transforming lipids and proteins.

The liver also synthesizes cholesterol and phospholipids. Phospholipids are, are the main part of lipoproteins. Phospholipids essentially make up most of the cell membranes we have. Our cholesterols are important for the body in many respects. If there’s too much cholesterol, however, the body will secrete it, either in bile, or cholesterol will be converted into bile acids because they have similar structure, and then it will be secreted in the intestines, and be excreted as feces.

Hope this video made sense. Thank you.

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